Ground water is used as the drinking water source in communities throughout out the world. Ground water is often cleaner than surface water because ground water typically contains lower levels of organic matter and pollutants. However, ground water can include many contaminants such as mercury, arsenic, fluoride, and lead.
Arsenic, for example, is a known carcinogen which is found in elevated concentrations in many ground water sources. In the United States, the Environmental Protection Agency has implemented a maximum concentration limit of 10 micrograms of arsenic per liter for drinking water. Therefore, it is often necessary to remove arsenic from drinking water.
An adsorption process is one method used to remove arsenic and other water soluble contaminants from a water source. Typically, the adsorption process contacts the contaminant-containing water with a solid media. The contaminant has greater affinity for the media than for the water. The contaminant bonds to the solid media and is removed from the water. Iron hydroxide, aluminum hydroxide, cupric hydroxide, titanium oxide, and manganese oxide are examples of contaminant-removal media. Periodically, the media is taken off-line and backwashed with water to reclassify the media bed and reduce flow restrictions in the bed.
In addition to contaminants, silicon dioxide (SiO2), also known as silica, is also present in many ground water sources. Silicon dioxide has been found to act as an interferent to the adsorption process. Silicon dioxide can reduce the media adsorptive capacity by 25% to 75%. It is believed that silicon dioxide polymerizes and forms a coating on the surface of the adsorption media. The silicon dioxide coating reduces the contact area between the contaminant-containing water and the pores in the adsorption media where adsorption occurs. Thus, the adsorptive capacity of the media is reduced. Consequently, it is desirable to remove the silicon dioxide from the adsorption bed to increase the adsorptive capacity of the bed.